Thinking Mathematically (6th Edition)

by
Blitzer, Robert F.

Answer

$1140$ ways

Work Step by Step

The order in which the members are chosen is not important
Combinations, ${}_{n}C_{r}=\displaystyle \frac{n!}{(n-r)!r!}$
${}_{20}C_{3}=\displaystyle \frac{20!}{(20-3)!3!}=\frac{20\times 19\times 18}{1\times 2\times 3}$
$=20\times 19\times 3=1140$